I am excerpting part of an answer to this question
the Aruch HaShulachan (as well as the B'er Heitev) in סימן קי - דיני תפילת ''הביננו'', ותפילת הדרך והפועלים says:
טו: אין לאמר תפילת הדרך אלא אם כן יש לו לילך פרסה, דפחות מפרסה לא נקרא ''דרך''. אלא אם כן הוא מקום סכנה, דאז גם בפחות מזה יש לאומרה.
"If one is in a dangerous area, then one says it even for less than a Parsa."
The NYC subway has some dangerous stations, especially late at night. Some subway lines themselves can be considered dangerous because of the neighborhoods they pass through and the people that board the train. I used NYC as an example, but I gather that similar evaluations may be made about many transit systems in the world.
The same source (Aruch Hashulchan) specifies that travel must be to another city. My daily commute on the subway is 1 hr. each way but is within the same city. Normally the distance would qualify, but since it's in the same city, I infer, no. However, the majority of my commute is through several dangerous neighborhoods. Based on this Aruch Hashulchan, should I say Tefillat Haderech? If so, should I say it twice daily, since I make 2 commutes?